Method and device for generating an oxygen product by low-temperature separation of air

ABSTRACT

The method and device serve for generating an oxygen product by low-temperature separation of air having a distillation column system for nitrogen-oxygen separation that comprises a high-pressure column ( 6 ) and a low-pressure column ( 7 ) which contain mass-transfer sections. Liquid that drains off from the lowest mass-transfer section ( 32 ) of the low-pressure column ( 7 ) is introduced into a main condenser ( 8 ) constructed as a bath evaporator and condenser-evaporator. Between the bottom end of the lowest mass-transfer section ( 32 ) of the low-pressure column ( 7 ) and the main condenser ( 8 ) there is arranged a liquid buffer ( 33 ). In the event of a reduction in load, liquid is introduced into the liquid buffer ( 33 ) and is stored there. In the event of an increase in load, liquid stored in the liquid buffer ( 33 ) is introduced ( 34 ) into the main condenser ( 8 ).

SUMMARY OF THE INVENTION

The invention relates to a method for generating an oxygen product by low-temperature separation of air using a distillation column system for nitrogen-oxygen separation comprising a high-pressure column and a lower-pressure column which contain mass-transfer sections. A feed air stream is cooled in a main heat exchanger and introduced into the high-pressure column. An oxygen-enriched product stream is taken off from the lower region of the low-pressure column, warmed in the main heat exchanger and obtained as oxygen product. Liquid that drains off from the lowest mass-transfer section of the low-pressure column is introduced into a main condenser, that is constructed as a bath evaporator and condenser-evaporator, and is partially vaporized in the main condenser. Non-vaporized liquid from the main condenser is introduced into a column bottom.

Methods and devices for low-temperature separation of air are known, for example from Hausen/Linde, Tieftemperaturtechnik [Low-temperature technology], 2nd Edition 1985, chapter 4 (pages 281 to 337).

The distillation column system of the invention can be formed as a two-column system (for example as a classical Linde double-column arrangement), or else as a three- or multi-column system. In addition to the columns for nitrogen-oxygen separation, it can comprise further devices for obtaining high-purity products and/or other air components, in particular noble gases, for example an argon production stage and/or a kryptonxenon production stage.

The low-pressure column has a lower operating pressure than the high-pressure column. For generating vapor, which ascends in its mass-transfer sections, the low-pressure column has a column-bottom evaporator which is termed the main condenser. This is constructed as a condenser-evaporator, that is to say, in indirect heat exchange with the vaporizing column-bottom liquid of the low-pressure column, a gaseous heating fluid is liquefied, for example, overhead nitrogen of the high-pressure column. The main condenser is frequently arranged directly within the low-pressure column (internal main condenser); alternatively it is accommodated in a separate vessel outside the low-pressure column and connected by pipelines to the low-pressure column (external main condenser).

Each “condenser-evaporator” has a liquefaction space and a vaporization space which contain liquefaction passages and vaporization passages, respectively. In the liquefaction space, the condensation of a first fluid stream is carried out, in the vaporization space, the vaporization of a second fluid stream is carried out. The two fluid streams are in indirect heat exchange in this case. The vaporization space and liquefaction space are formed by groups of passages which are in a heat-exchange relationship with one another.

The main condenser can be constructed as a falling-film evaporator or as a bath evaporator. The present invention relates to air separation methods in which the main condenser is constructed as a bath evaporator. In a “bath evaporator” (occasionally also called “circulation evaporator” or “thermosiphon evaporator”), the heat-exchanger block is in a liquid bath of the fluid that is to be vaporized. This flows, owing to the thermosiphon effect, from bottom to top through the vaporization passages and exits again at the top as a two-phase mixture. The remaining liquid flows outside the heat-exchanger block back into the liquid bath. (In a bath evaporator, the vaporization space can comprise not only the vaporization passages but also the exterior space around the heat-exchanger block). In a falling-film evaporator, in contrast, additional measures are necessary in order to force the liquid through the vaporization passages.

As main condenser, two or more bath evaporators arranged next to one another can also be used, which are then connected in parallel on the vaporization side and liquefaction side. Each of these bath evaporators, or the single bath evaporator, which forms the main condenser, can be constructed in a single-storey or multistorey manner. A “multistorey bath evaporator” has two or more storeys arranged one above the other, each of which is implemented by a heat-exchanger section. In this case, each individual storey can be implemented by a separate heat-exchanger block, or at least two, or else all, storeys are formed by sections of a shared heat-exchanger block. The storeys can be connected together in series or parallel both on the vaporization side and on the liquefaction side.

A special embodiment of a multistorey bath evaporator is a “cascade evaporator”. Here, the storeys on the vaporization side are connected together in series, that is to say non-vaporized liquid flows from an upper storey further to the storey therebelow. On the liquefaction side, cascade evaporators are preferably likewise connected in series, for example by liquefaction passages of a shared heat-exchanger block passing through all of the storeys. Alternatively, in a cascade evaporator, the storeys can also be connected together in parallel on the liquefaction side.

The “main heat exchanger” can be formed from one or more heat-exchanger sections connected in parallel and/or series, for example from one or more plate heat-exchanger blocks.

Processes using single-storey bath evaporators are known from the abovementioned monograph by Hausen/Linde. Methods of the type mentioned at the outset using multistorey bath evaporators are disclosed in DE 1152432, DE 1949609 A, WO 0192798 A2, EP 1287302 B1 and DE 102007003437 A1.

An object of the invention is to specify such a method and a corresponding device which permit particularly stable operation of the system, in particular in the case of rapid changes in load.

Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

A change in load is an operating case in which the plant is in a non-steady-state transition phase of oxygen products from a first production rate to a second production rate. In an “increase in load”, the second production rate is higher than the first, in a “reduction in load” it is lower.

These objects are achieved by method and apparatus according to the invention. In accordance with the invention, a liquid buffer is arranged above the main condenser. This liquid buffer, during the steady-state operation of the method, can be filled with a suitable liquid, for example with some of the reflux liquid which drains off from the lowest mass-transfer section of the low-pressure column. The liquid buffer is filled slowly, for example during the steady-state operation, in such a manner that the buffered liquid is available for the non-steady-state operation in the case of an increase in load. Furthermore, in accordance with the invention, during a reduction in load, liquid from the column bottom of the main condenser is introduced into the liquid buffer and in this case the storage contents of the liquid buffer are increased, that is to say in total, more liquid is introduced into the liquid buffer than is even taken off therefrom.

During an increase in load, in contrast, liquid from the liquid buffer is introduced into the main condenser and in this case the storage contents of the liquid buffer are decreased, that is to say more liquid is taken off from the liquid buffer than is introduced into it.

In principle, a bath evaporator, in comparison with the falling-film evaporator, has the advantage that no such external liquid circulation is necessary. The use of such an artificial liquid circulation from the column bottom of the main condenser to the buffer therefore first appears to be nonsensical. In the context of the invention, however, it has proved that the operating advantage is surprisingly so great that it justifies the corresponding additional expenditure.

During a reduction in load, the system according to the invention, in addition, has the advantage that the relative high amount of liquid which in this case drains off from the upper mass-transfer sections of the low-pressure column with low purity can at least in part be collected in the buffer and therefore the contamination of the column bottom liquid is prevented or reduced.

In non-steady-state operating conditions, for example during an increase in load, frequently the efficiency of the heat-exchange process at the main condenser is decreased. In the context of the invention, it was found that this is due to a liquid shortage on the vaporization side. Using the method according to the invention, liquid that is lacking can now be supplemented from the buffer. A particularly stable and interference-free operation of the plant is thereby possible even under extreme operating conditions, for example in the case of a rapid change in load with a changing rate of more than one percent change in load per minute.

The liquid buffer is arranged below the lowest mass-transfer section of the low-pressure column and above the main condenser, that is to say in such a manner that liquid, owing to the natural gradient, can flow from the buffer into the vaporization space of the main condenser or the uppermost storey thereof. The liquid buffer can be formed, for example, by one or more dishes which are arranged on the column wall, for example by an encircling dish, or else by one or more chimney trays.

The “column bottom” of a single-storey bath evaporator is generally formed by its liquid bath. In the case of a multistorey bath evaporator, the lowest liquid bath is generally operated as a column bottom. Alternatively, the column bottom can be formed by a separate space below the main condenser.

The liquid circulation from the column bottom to the liquid buffer can also be used in the steady-state operating case, without the storage contents necessarily increasing. In this case, in bath evaporators, the otherwise unusual liquid circulation serves for equalizing purity differences in the vaporizing liquid over the height of the evaporator. In particular in the case of multistorey bath evaporators, a particularly stable operation can be achieved thereby.

In a further embodiment of the method according to the invention, the liquid from the column bottom of the main condenser is introduced by a liquid pump into the liquid buffer.

Since the liquid buffer is arranged above the main condenser, and the column bottom, however, is arranged thereunder or at the lower end, the column bottom liquid must be lifted in order to arrive at the liquid buffer. In principle, for this purpose, any method for lifting a liquid can be used. In particular, a liquid pump is used.

The invention offers particular advantages in the application to multistorey bath evaporators. The greatest is the advantage when the main condenser is constructed as a cascade evaporator. Here, owing to the liquid circulation from the column bottom to the buffer, not only is a possible liquid deficiency prevented, but, in addition, the concentration differences on the vaporization side of the various storeys (stages) are compensated for. This is because each stage of a cascade evaporator acts as a partial vaporization, that is to say from top to bottom the oxygen concentration increases and thereby the vaporization temperature. On the liquefaction side, however, the same nitrogen flows everywhere with virtually constant liquefaction temperature. As a result the upper stages of a cascade evaporator operate in principle with a greater temperature difference than the lower stages. During changes in load, this increases the deficiency of liquid in the upper stages further, because these convert more heat than the lower stages. In the context of this embodiment of the invention, the purest liquid which flows out of the lowest stage into the column bottom is now conducted back upwards into the buffer and from there to the topmost vaporization stage. It thereby compensates the relatively low purity there and generates overall a flatter concentration profile on the vaporization side over the height of the cascade evaporator. It therefore simultaneously counteracts both adverse effects during operation of a cascade evaporator, namely the liquid deficiency during changes of load and the unwanted concentration differences over the height of the main condenser.

The use of the method according to the invention is particularly expedient when the oxygen-enriched product stream has an oxygen concentration of less than 98%, for example 90 to 95% (all percentages relate here and hereinafter to the molar amount, unless stated otherwise).

The method can be operated, in particular, by two modes of operation. In a first mode of operation, more liquid is introduced into the liquid buffer than is taken off therefrom. In a second mode of operation, more liquid is taken off from the liquid buffer than is introduced thereinto. The first mode of operation corresponds, for example, to a steady-state operation with constant load, the second mode of operation to a case of change in load, for example an increase in load during the transition time from a first steady-state operation having a first production rate to a second steady-state operation having a second, higher production rate.

The invention, furthermore, relates to a device for low-temperature separation of air in accordance with the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawing wherein:

the FIGURE illustrates an exemplary embodiment of the invention.

Compressed and purified feed air 1 flows at a pressure of approximately 5.5 bar into the warm end of a main heat exchanger 2, more precisely branched into a first air substream 3 and a second air substream 4. Some of the dry air 1 can be branched off via line 1A as instrument air or for supplying other compressed air consumers.

The first air substream 3 is cooled in the main heat exchanger 2 to about dew point and introduced via line 5 into the high-pressure column 6 which is part of a two column distillation system (such as a double column arrangement as shown in the FIGURE) for nitrogen-oxygen separation that in addition has a low-pressure column 7 and a main condenser 8 that is constructed as a cascade evaporator. The operating pressures in the columns (in each case at the top) are approximately 5.2 bar in the high-pressure column 6 and approximately 1.3 bar in the low-pressure column 7.

The second air substream 4 is only cooled to an intermediate temperature in the main heat exchanger 2 and at this intermediate temperature is passed to an expansion turbine 9 which is braked by a generator 10. There, it is work-producingly expanded to about the pressure of low-pressure column 7, passed via line 11 back to the main heat exchanger 2 and finally via line 12 to the low-pressure column 7 at an intermediate point.

Gaseous overhead nitrogen 13 removed from the high-pressure column 6 is in part 14 introduced into the liquefaction space of the main condenser 8. The remainder 15 is warmed in the main heat exchanger 2 to about ambient temperature and finally taken off via line 16 as gaseous nitrogen pressurized product (PLAN).

A first part 18 of the liquid nitrogen 17 generated in the main condenser 8 is applied as reflux to the top of the high-pressure column 6. A second part 19 is cooled in a subcooling-counterflow heat exchanger 23 and applied via line 20 as reflux to the top of the low-pressure column 7. If required, a third part 21 can be removed as liquid nitrogen product (LIN).

The oxygen-enriched column bottom liquid 24 of the high-pressure column 6 is likewise cooled in the subcooling-counterflow heat exchanger 23 and introduced via line 25 into the low-pressure column 7 at an intermediate point.

From the top of the low-pressure column 7, gaseous nitrogen 26 is taken off, warmed in the subcooling-counterflow heat exchanger 23 and in the main heat exchanger 2 and taken off via line 27. It can be used, for example, as regeneration gas in a feed air purification step (not shown).

From the bottom region of the low-pressure column (here directly above the main condenser 8), an oxygen-enriched product stream 28 is taken off, warmed to about ambient temperature in the main heat exchanger 2 and obtained as oxygen product (GOX) via line 29. Via lines 30, 31, 33 and a pump 32, some of the liquid from the column bottom 35 of the main condenser 8 can be obtained as liquid oxygen product (LOX), for example for filling a liquid tank for emergency supply.

Alternatively, or additionally, to the gaseous product withdrawal, a pressurized oxygen product could be obtained by internal compression by bringing some of the liquid oxygen 31 to pressure in the liquid state and vaporizing or pseudo-vaporizing it in the main heat exchanger 2.

According to the invention, directly below the lowest mass-transfer section 38 of the low-pressure column 7 there is situated a liquid buffer 33 in the form of an annular dish. During the steady-state operation of the plant, this can be filled with some of the liquid which drains off from the lowest mass-transfer section 32. Via line 34, in the case of an increase in load, liquid is passed from the buffer 33 in a targeted manner into the main condenser 8, more precisely into the liquid bath of its uppermost storey.

In any case, in the event of a reduction in load via a liquid pump 36 and line 37, liquid from the column bottom 36 is introduced into the liquid buffer 33.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding European Application No. 11002364.5, filed Mar. 22, 2011 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. 

1. A method for the variable generation of an oxygen product by low-temperature separation of air using a distillation column system for nitrogen-oxygen separation that comprises a high-pressure column (6) and a lower-pressure column (7), which each contain mass-transfer sections, said method comprising: cooling a feed air stream (1, 3, 5) in a main heat exchanger (2) and introducing the cooled feed air stream into a high-pressure column (6), removing an oxygen-enriched stream (28) from the lower region of a low-pressure column (7), warming the oxygen-enriched stream in said main heat exchanger (2), and obtaining warmed the oxygen-enriched product stream as oxygen product (29), introducing liquid drained off from the lowest mass-transfer section (32) of said low-pressure column (7) into a main condenser (8) that is constructed as a bath evaporator and condenser-evaporator, and partially vaporizing said liquid drained off from the lowest mass-transfer section (32) in said main condenser, introducing non-vaporized liquid from the main condenser (8) into a column bottom (35), and providing a liquid buffer (33), constructed for storage of liquid, between the bottom end of said lowest mass-transfer section (32) of said low-pressure column (7) and the main condenser (8), wherein during a reduction in load, liquid from said column bottom (35) of said main condenser is introduced into said liquid buffer (33) and is stored there and, thereby the storage contents of said liquid buffer (33) are increased, and wherein during an increase in load, at least some of the liquid stored in said liquid buffer (33) is introduced (34) said main condenser (8) and, thereby the storage contents of said liquid buffer (33) are decreased.
 2. The method according to claim 1, wherein the liquid from said column bottom (35) of said main condenser is introduced by a liquid pump (36) into said liquid buffer (33).
 3. The method according to claim 2, wherein said main condenser (8) is constructed as a multistorey bath evaporator.
 4. The method according to claim 3, wherein said main condenser (8) is constructed as a cascade evaporator.
 5. The method according to claim 3, wherein said oxygen-enriched stream (28) has an oxygen concentration of less than 98%.
 6. The method according to claim 4, wherein said oxygen-enriched stream (28) has an oxygen concentration of less than 98%.
 7. An apparatus for generating an oxygen product by low-temperature separation of air, said apparatus comprising: a distillation column system for nitrogen-oxygen separation comprising a high-pressure column (6) and a low-pressure column (7), which each contain mass-transfer sections, a main heat exchanger (2) for cooling a feed air stream (1, 3, 5), means for introducing cooled feed air stream from said main heat exchanger into said high-pressure column (6), means for removing an oxygen-enriched stream (28) from the lower region of said low-pressure column (7) and means for introducing the oxygen-enriched stream (28) into said main heat exchanger (2), an oxygen product line for obtaining the warmed product stream as oxygen product (29) from said main heat exchanger (2), a main condenser (8) constructed as a bath evaporator and condenser-evaporator, means for introducing liquid that drains off from the lowest mass-transfer section (32) of said low-pressure column (7) into said main condenser (8) a column bottom (35) for collecting non-vaporized liquid from said main condenser (8), a liquid buffer (33) arranged between the bottom end of said lowest mass-transfer section (32) of said low-pressure column (7) and said main condenser (8), means for introducing liquid into said liquid buffer (33), means for introducing (34) at least some of the liquid stored in said liquid buffer (33) into said main condenser (8), and control means constructed in such a manner that during a reduction in load, liquid from said column bottom (35) of said main condenser is introduced into said liquid buffer (33) and is stored there and, in this case, the storage contents of said liquid buffer (33) are increased, and that during an increase in load, at least some of the liquid stored in said liquid buffer (33) is introduced (34) into said main condenser (8) and, in this case, the storage contents of said liquid buffer (33) are decreased.
 8. The apparatus according to claim 7, further comprising a liquid pump (36) for transporting liquid from said column bottom (35) of said main condenser into said liquid buffer (33).
 9. The apparatus according to claim 8, wherein said main condenser (8) is constructed as a multistorey bath evaporator.
 10. The apparatus according to claim 8, wherein said main condenser (8) is constructed as a cascade evaporator. 